1. Field of Invention
This invention applies to semiconductor testing and in particular to burn-in testing and the monitoring of product bias during burn-in.
2. Description of Related Art
Burn-in testing is an important means by which infant mortality and early life failures can be culled out from a semiconductor product line. At the same time it is important that the product be tested under bias stress during burn-in. The stress testing should not be to much leading to unnecessary failures or too little and not finding enough failing parts to assure an outgoing product quality level.
The applied bias to the semiconductor parts during burn-in provides the necessary electrical stress and is usually at a voltage higher than for normal operations. Shifts in this voltage, commonly called "noise", provides a source of the product being over stressed or under stressed during test. This potential for bias voltage variation as seen by the product under test gives rise to the need for a means to monitor the applied voltage.
In U.S. Pat. No. 5,452,253 (Choi) an internal voltage generator provides a regulated bias for a semiconductor memory array. When a burn-in stress voltage is detected, the internal voltage generator provides an voltage stress to the semiconductor array. The burn-in test mode is activated if particular signal conditions are met and the external voltage is higher than a pre-set burn-in test voltage. This prevents the test device from entering test mode due to noise on the external power supply.
In U.S. Pat. No. 5,561,639 (Lee et al.) a burn-in test signal generator detects whether the applied voltage bias to a semiconductor memory chip is normal or at a burn-in level. When a burn-in level voltage is detected, test time can be reduced by selecting a plural of memory cell rows in accordance with any address applied during burn-in and different from normal operations.
In U.S. Pat. No. 5,656,944 (Choi) a burn-in detection circuit detects whether an external applied voltage is above a predetermined voltage level for burn-in testing and having a hysteresis effect in which coming out of burn-in test is not allowed until the applied external voltage further increases before decreasing. This in turn prevents noise on the external voltage from taking the test device out of burn-in.
Whereas, the above references concentrate on providing a burn-in bias and attempting to avoid noise on the applied external voltage bias, it is important to know what the external voltage bias is throughout the burn-in test. Determining that the voltage bias that is stressing the product is too high could indicate that there were parts that failed which should have passed the test. At the same time an under voltage provides an under stress of the product and could provide tested parts that have too high a failure rate.